"We can't believe that, it's unbelievable," says Abu Walid, another medical student and friend of the three men, describing his initial reaction to their deaths. "Then we swore to continue my friends' way, to protect the future Syria from Assad. He's a criminal president."

Those sentiments were voiced by a number of medical volunteers, many of them students, during a three-day visit by the Monitor to the rebel-held Aleppo enclave of Salaheddin.

"They wanted by this crime to tell us that everyone who works like this will meet the same destiny, but we are insisting on our duty more," says Hamza, the team leader of the three who were killed. "If we were afraid, the revolution would have stopped from the very beginning."

'We will kill Assad for killing him'

Amnesty International detailed the evidence of torture and described the killings of the medical workers in a June report, as "yet more evidence that Syrian government forces are prepared to commit unspeakable crimes to silence dissent."

Even before the rebels took root in Aleppo – Syria's most populous city, northern commercial hub, and in the past largely pro-regime – the government "intensified its hunt for the wounded and for those who provide life-saving emergency treatment to them" across the country, Amnesty said. Such regime violations were part of an "increasingly entrenched pattern of crimes against humanity."

Abu Walid, who wore a blue T-shirt and shorts in the heat of the field hospital, said the three men had been helping patients in Salaheddin on June 17. They left for home around midnight and were stopped at a checkpoint along the way. Medical instruments were found in the car.

When Mr. Aslan's aunt called her nephew's mobile phone a couple hours later, it was answered by man who said, "Don't ask about them, we will teach them."

The three were in the hands of Air Force Intelligence, and not found for a week. They were only recognizable by pieces of clothing, like a belt, or body shape. Amnesty reported that their identity cards had been left beside the corpses.